1. Field of the Invention
The invention relates to an X-ray examination apparatus, comprising an X-ray source, an X-ray detector and an X-ray filter which is arranged between the X-ray source and the X-ray detector and includes a plurality of filter elements whose X-ray absorptivity can be adjusted by adjustment of a quantity of X-ray absorbing liquid within individual filter elements.
2. Description of Related Art
An X-ray examination apparatus of this kind is known from U.S. Pat. No. 5,625,665.
The X-ray examination apparatus forms an X-ray image of an object to be examined, for example a patient to be radiologically examined. The X-ray source irradiates the object by means of an X-ray beam and an X-ray image is formed on the X-ray detector due to local differences in the X-ray absorption within the object. The X-ray filter ensures that the range of brightness values of the X-ray image remains limited. The X-ray filter is adjusted in such a manner that on the one hand parts of the X-ray beam which are only hardly attenuated by the object are slightly attenuated by the X-ray filter and that on the other hand parts of the X-ray beam which are significantly attenuated by the object are transmitted by the filter substantially without attenuation. Because the brightness values of the X-ray image lie within a limited range, the X-ray image can be readily further processed so as to reproduce small details of low contrast clearly.
The X-ray filter of the known X-ray examination apparatus includes a bundle with a very large number of capillary tubes, each of which communicates with the X-ray absorbing liquid by way of one end. The adhesion of the X-ray absorbing liquid to the inner wall of such a capillary tube is dependent on the electric voltage applied to the relevant capillary tube. Specifically, the contact angle enclosed by the X-ray absorbing liquid relative to the inner wall of such a capillary tube is dependent on the electric voltage difference between the inner wall and the X-ray absorbing liquid. When the contact angle is larger than 90.degree., the X-ray absorbing liquid hardly penetrates such a capillary tube and when the contact angle is smaller than 90.degree., such a capillary tube is filled with a quantity of X-ray absorbing liquid which is dependent on the magnitude of the electric potential difference. The capillary effect of the capillary tubes in respect of the X-ray absorbing liquid can thus be electrically controlled. The quantity of X-ray absorbing liquid in each of the capillary tubes is controlled on the basis of the electric voltages applied to the capillary tubes.
The manufacture of the X-ray filter of the known X-ray examination apparatus is intricate and hence expensive, because it is very difficult to bundle a very large number of capillary tubes in a regular pattern.